Fix an issue and harden a few spots

1.  Real issue was that runtime.nothing was not defined, and string-dict used
it without any actual tests for set-now returning nothing

2.  checkConstructorArgs really ought to not ask isRefGraphable, but should ask
isGraphableRef.  That's an unfortunate naming that could also be remedied.  For
now, make sure that we check that the value is actually a ref before trying to
look up its state field by calling the latter instead.